This invention is in the field of tribology, more specifically formulations which provide good antiwear and beneficial friction properties.
Ionic liquids (ILs) are a new generation of antiwear additives that are superior to traditional antiwear additives such as ZDDP. Ionic liquids are most commonly defined as organic salts with melting points or glass transition temperature below 100° C. Although this description gives a clear idea of their ionic nature and their liquid state at a relatively low temperature, it is worthwhile to stress the importance of ILs in comparison to molten salts. Usually, fusion temperature of a salt is considerably high, for example 801° C. in the case of sodium chloride, which excludes its use in many applications. However, by the use of ILs, it is possible to benefit from properties emerging from ionic bonds between the moieties, but at a relatively low temperature, often significantly below room temperature. The provision and maintenance of these properties are in particular important for their use as lubricants to enable application over a wider range of temperature.
In order to decrease the melting temperature, ILs are generally constituted from an organic cation with low symmetry and a weakly coordinated anion. This way, lattice energy is lower and the anion-cation interaction is minimized due to the asymmetric and delocalized charge.
The reason for the growing interest in ILs can be explained by their excellent physical-chemical properties such as their large electrochemical window, controlled miscibility, high thermal stability, negligible vapor pressure, and in some cases, environmental harmlessness. In addition to these qualities, it is possible to obtain compounds with tailor-designed properties by tuning the structure through substitution and structural modification of the anion or of the cation. For example, changes in the anion can influence the chemical behavior and the stability of the IL while the use of different cations can affect physical properties, such as viscosity, melting point, and density.
In 2001, Ye et al. performed the first tribological investigation with ILs. C. Ye, W. Liu, Y. Chen, L. Yu: “Room-temperature ionic liquids: a novel versatile lubricant”. Chem. Commun., (2001), 2244-2245. This research group used imidazolium tetrafluoroborate as a lubricant for various tribo-pairs and, in all the experiments, the use of ILs showed significant friction reduction. After this initial research, many other researchers have studied the tribological behavior of ILs and the number of chemical structures investigated and papers published on this topic grows rapidly every year. In addition, dicationic ILs (DILs) have been investigated for their tribological behavior and showed good performances. The thermal stabilities of DILs are generally greater than those of most traditional monocationic ILs.
Zinc dialkyl dithiophosphates (ZDDPs) are the most common additives used in hydraulic, gear, and engine oils. The use of ZDDPs, however, presents disadvantages. For example, ash generation by ZDDPs is dangerous for engine oils, since it reduces significantly the durability of the after treatment system installed in the exhaust system to reduce undesired emissions, mainly carbon monoxide, unburned hydrocarbons, and oxides of nitrogen, generated in the engine.
Ashless thiophosphates also have been shown to exhibit superior wear performance, and have been shown to be superior to ZDDP in some aspects. U.S. Pat. Nos. 7,074,745 and 8,216,982 and Publication No. 2011/0319303 disclose ashless fluorothiophosphates. In addition, alkylthioperoxydithiophosphates are described in U.S. patent application Ser. No. 13/887,968, filed on May 6, 2013.